A single loudspeaker may create sound at both ears of a listener. For example, a loudspeaker on the left side of a listener will still generate some sound at the right ear of the listener along with sound, as intended, at the left ear of the listener. The objective of a crosstalk canceler is to allow production of sound from a corresponding loudspeaker at one of the listener's ears without generating sound at the other ear. This isolation allows any arbitrary sound to be generated at one ear without bleeding to the other ear. Controlling sound at each ear independently can be used to create the impression that the sound is coming from a location away from the physical loudspeaker (i.e., a virtual loudspeaker/sound source).
In principle, a crosstalk canceler requires only two loudspeakers (i.e., two degrees of freedom) to control the sound at two ears separately. Many crosstalk cancelers control sound at the ears of a listener by compensating for effects generated by sound diffracting around the listener's head, commonly known as Head Related Transfer Functions (HRTFs). Given a right audio input channel xR and a left audio input channel xL, the crosstalk canceler may be represented as:
      [                                        y            L                                                            y            R                                ]    =                    [        H        ]            ⁡              [        W        ]              ⁡          [                                                  x              R                                                                          x              L                                          ]      
In this equation, the transfer function H of the listener's head due to sound coming from the loudspeakers is compensated for by the matrix W. Ideally, the matrix W is the inverse of the transfer function H (i.e., W=H−1). In this ideal situation in which W is the inverse of H, sound yL heard at the left ear of the listener is identical to xL and sound yR heard at the right ear of the listener is identical to xR. However, many crosstalk cancelers suffer from ill-conditioning at some frequencies. For example, the loudspeakers in these systems may need to be driven with large signals (i.e., large values in the matrix W) to achieve crosstalk cancellation and are very sensitive to changes from ideal. In other words, if the system is designed using an assumed transfer function H representing propagation of sound from the loudspeakers to the listener's ears, small changes in H can cause the crosstalk canceler to achieve a poor listening experience for the listener.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.